Polyethers used for producing polyurethanes, i.e. meeting the most rigid quality requirements, especially as far as uniformity is concerned with respect to molecular-weight distribution and according to the functionality of the product, can be obtained by a continuous method following known technology.
USSR Inventor's Certificate No. 244,612 describes a process of anionic polymerization run in a counter-flow column apparatus upon contacting a solution of catalyst (for example, glycerate of an alkaline metal in a polyhydric alcohol) delivered in a downward direction (concentration of an alkaline agent is determined by the grade of polyether obtained) with alkylene oxide vapors which are fed into the apparatus from the bottom in excess (usually 20-50%) with respect to the required stoichiometric amount.
The disadvantages of known method are as follows:
(1) Development of industrial column apparatus presents serious problems since the apparatus should possess such incompatible characteristics as laminarity of the downward flow of the liquid phase and uniform stirring thereof in the cross-sectional direction for leveling the reagent concentration and temperature under conditions of exothermal reaction of alkylene oxide polymerization when elimination of excess heat is required, for example, directly through the apparatus walls. The contadictoriness of these requirements becomes more pronounced with increasing size of the apparatus.
(2) Feeding the liquid phase into the apparatus from above always causes a possible "jump" of the part of the liquid through the reaction zone which leads to the appearance of low molecular weight by-products in the final polyether. This possibility increases when alkoxides of alkaline metals and their solutions in polyhydric alcohols are used, with a high content of an alkaline agent (10-20% as calculated for KOH), due to a difference in density of the initial (low-molecular weight) and intermediate (polyether of various molecular weight) products.
(3) High viscosity and density of the initial alkoxide solution and incompatibility thereof with alkylene oxide yield a heterogeneous system at the initial step of the process as a result of which polymerization proceeds only at the interface, which is small when viscosity is high and stirring of the liquid phase inefficient. This calls for a considerable increase in size of that part of the apparatus where the first step of the process takes place.
A second known method, that of U.S. Pat. No. 3,117,998 to Cosby is characterized by the process of anionic polymerization is run in series-connected direct-flow hollow apparatuses upon contacting the initial polyhydric alcohol fed continuously into the bottom part of the first apparatus, alkylene oxide introduced continuously into the bottom part of each of the apparatuses (in 25-150% excess by weight of the oxide reacting in each apparatus), and solid alkali (KOH) introduced intermittently into the first or into the rest of the apparatuses as well from above on a perforated grate (or net) immersed into the reaction mixture. This method is free from disadvantages inherent in first one cited above but has its own drawbacks:
(1) According to this method, only alkali and not alkoxide must be used as a catalyst.
Alkali in the reaction mixture reacts with alkylene oxide to inevitably form diols; polyethers uniform in functionality cannot be produced according to said method for average functionality higher than 2.
(2) Stepwise addition of solid alkali requires constant operator control and causes fluctuations in the catalyst concentration during the reaction; all this affecting the main process kinetics. Furthermore, the unit for feeding alkali must be too bulky to meet sealing requirements for the whole apparatus.
(3) A small amount of series-connected apparatuses (4-6), in spite of the large height, as compared with diameter, cannot ensure short residence time of the reagents in the reaction zone. As a result the final product contains both low- and high-molecular weight components.
(4) Starting the apparatus requires each column to be previously filled with intermediates having an appropriate average molecular weight, each intermediates being obtained in a batch reactor.
It is an object of the invention to eliminate the above disadvantages. Another object of the invention is to provide a continuous method of producing polyethers more uniform in chemical composition (molecular-weight distribution and distribution with respect to functionality types).
It is also an object of the invention to provide a method of producing polyethers whose realization does not require complex technological and auxiliary equipment.
Still another object of the invention is to provide a method of producing polyethers whose technological parameters, especially at the initial step, can be easily and simply regulated.